A systematic study of the variation of tetrathiafulvalene (TTF), TTF+ and TTF2+ reaction pathways with water in the presence and absence of light

Article abstract of DOI:10.1039/c4ra08038f

The chemistry of the strongly electron donating tetrathiafulvalene (TTF) molecule is exceptionally well known, but detailed knowledge of the chemistry of its technologically important one (TTF⁺) and two (TTF²⁺) electron oxidised redox partners is limited. In this paper, the different pathways that apply to the reaction of TTF, TTF⁺ and TTF²⁺ with water have been identified in the absence and presence of light. On the basis of data obtained by transient and steady state voltammetric methods in CH₃CN (0.1 M Bu₄NPF₆) containing 10% (v/v) H₂O, TTF is shown to participate in an acid base equilibrium reaction with HTTF⁺, with H₂O acting as the proton donor. In contrast, TTF⁺ generated by one electron bulk oxidative electrolysis of TTF remains unprotonated and fully stable in the presence of 10% H₂O in the dark. However, when this cation radical is exposed to white or blue (λ = 425 nm) light, TTF⁺ is photoreduced to TTF, with oxidation of water to give oxygen (detected by a Clark electrode) and protons that react with TTF to give HTTF⁺ as the counter reaction. Again emphasising important reaction pathway differences associated with each redox level, TTF²⁺ generated by bulk two electron oxidative electrolysis of TTF reacts rapidly with water, even in the dark, to give TTF⁺, protons, HTTF⁺ and oxygen as the products.

Full text of DOI:10.1039/c4ra08038f

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A systematic study of the variation of
tetrathiafulvalene (TTF), TTF+c and TTF2+ reaction
pathways with water in the presence and absence
of light†
Cite this: RSC Adv., 2014, 4, 49789
Shaimaa M. Adeel, Qi Li, Ayman Nafady,‡ Chuan Zhao,§ Amal I. Siriwardana,{
*
*
Alan M. Bond and Lisandra L. Martin
The chemistry of the strongly electron donating tetrathiafulvalene (TTF) molecule is exceptionally well
known, but detailed knowledge of the chemistry of its technologically important one (TTF+c) and two
(TTF2+) electron oxidised redox partners is limited. In this paper, the different pathways that apply to the
reaction of TTF, TTF+c and TTF2+ with water have been identified in the absence and presence of light.
On the basis of data obtained by transient and steady state voltammetric methods in CH3CN (0.1 M
Bu4NPF6) containing 10% (v/v) H2O, TTF is shown to participate in an acid base equilibrium reaction with
HTTF+, with H2O acting as the proton donor. In contrast, TTF+c generated by one electron bulk oxidative
electrolysis of TTF remains unprotonated and fully stable in the presence of 10% H2O in the dark.
However, when this cation radical is exposed to white or blue (l ¼ 425 nm) light, TTF+c is photoreduced
to TTF, with oxidation of water to give oxygen (detected by a Clark electrode) and protons that react
with TTF to give HTTF+ as the counter reaction. Again emphasising important reaction pathway
differences associated with each redox level, TTF2+ generated by bulk two electron oxidative electrolysis
of TTF reacts rapidly with water, even in the dark, to give TTF+c, protons, HTTF+ and oxygen as the products.
Received 3rd August 2014
Accepted 19th September 2014
DOI: 10.1039/c4ra08038f
www.rsc.org/advances
sensors, and organic eld-effect transistors.11–18 So, despite it
being more than forty years19 since the rst semi-conducting,
organic charge-transfer salt, TTF–TCNQ, (TCNQ ¼ 7,7,8,8-tet-
1. Introduction
Tetrathiafulvalene (TTF) and its derivatives have attracted
intense interest over a considerable time due to the i.e. strong
electron donor ability as well as optical properties. These redox
active molecules show a propensity to form strong H-bonds,
p-stack to give stable radical dimers providing facile routes to
novel molecular (and supramolecular) architectures, electrically
conducting organic materials, and stable charge-transfer
compounds.1–7 Some applications of derivatised TTF including
functionalization with C60 creating self-assembled monolayers,8
quinoxaline for solar cells9 and supramolecular assemblies10
have been of enormous interest. It is therefore not surprising
that TTF-derived materials have enabled a diverse range of
technological applications, for example in nonlinear optics,
racyanoquinodimethane) was reported, there remains a paucity
of detailed mechanistic data and knowledge on the underlying
chemistries for the one and two electron oxidised TTF+c and
TTF2+ species.
Few studies have explored the chemistry of TTF with water,
light and with oxygen. Notably, Girault et al.20 reported the four-
electron reduction of oxygen to water by TTF using an acidied
mixture of H2O–dichloromethane solution. A report by Ndama-
nisha et al. in which a TTF–carbon composite was prepared to
increasetheelectrondonorabilityofTTFwas alsofoundtoreduce
oxygen to water.21 Clearly, as TTF is such a redox active molecule,
electrochemical techniques are invaluable tools to investigate the
reactions of TTF, TTF+c and TTF2+ especially with water, light and/
or oxygen as reactants. In most organic solvents, TTF undergoes
two well-separated, one-electron, chemically and electrochemi-
cally reversible oxidation processes, to generate the monocation
radical, TTF+c and with further oxidation, the dication
(TTF2+)2,22–25 as described in eqn (1) and (2);
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia. E-mail:
Lisa.Martin@monash.edu; Alan.Bond@monash.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08038f
‡ Current address: King Saud University, College of Science, Department of
Chemistry, PO Box 2455, Riyadh 11451, Saudi Arabia.
TTF # TTF+c + e
(1)
(2)
§ Current address: School of Chemistry, The University of New South Wales,
Sydney, NSW 2052, Australia.
TTF+c # TTF2+ + e.
´
´
{ Current address: Tecnalia, Parque Tecnologico de San Sebastian, Mikeletegi
´
Pasealekua, 2, E-20009 Donostia-San Sebastian, Gipuzkoa, Spain.
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Thus, in acetonitrile, bulk oxidative electrolysis of TTF can anhydrous CHCl3 under nitrogen and added slowly to the TTF
quantitatively generate TTF+c and TTF2+ and these products can solution at room temperature. This mixture was stirred for 1
be exposed to water and light in order to establish the reactivity hour aer which the desired product was obtained as a dark-red
of each redox level under relevant conditions. Acid–base powder. The crude solid was recrystallized from anhydrous
chemistry of the TTF and its cations is also an important CH3CN at ꢀ20 ꢁC which afforded ne dark red needles. The
consideration. Giffard et al.26 showed that TTF can be modied composition was conrmed to be (HTTF)BF4 by 1H NMR
in acid media by protonation of the central C]C double spectroscopy.26
bond27–30 to give HTTF+ (eqn (3)). This protonated species can
also undergo reaction with another TTF molecule to give two
Electrochemistry
TTF-based radicals, via
a
radical–substrate coupling, as
described in eqn (4).
Cyclic voltammetric measurements were undertaken at 22 ꢂ
1 ꢁC with a BAS100 electrochemical workstation interfaced to a
standard three-electrode electrochemical cell conguration.
Glassy carbon (GC) (3 mm or 1 mm diameter), gold (Au, 1 mm
diameter), platinum (Pt, 1.6 mm), carbon bre (C, 7 mm or 12
mm diameter) disks or indium tin oxide (ITO)-coated glass with
(3)
(4)
Here we provide a detailed investigation of the inuence of an area of 0.06–0.1 cm2 and 8–12 U sqꢀ1 sheet resistance,
water and light on TTF0/+ /2+
chemistry in the CH3CN–H2O obtained from Delta technologies, were used as the working
c
(90 : 10) solvent mixture. In particular, reactions of TTF and the electrodes. GC, Au and Pt electrodes were polished with 0.3 mm
TTF+c and TTF2+ cations, generated electrochemically, have aqueous alumina slurries, rinsed with distilled water, acetone
been studied by voltammetric and spectroscopic techniques in and then dried with nitrogen gas prior to each measurement.
CH3CN (0.1 M Bu4NPF6) solution containing deliberately added ITO electrodes were cleaned by sonication in acetone and then
water under dark conditions or when exposed to light. Reac- 2-propanol for 5 min prior to use. Ag/Ag+ (10 mM AgNO3 in
tions of interest that have been established include protonation CH3CN, 0.1 M Bu4NPF6) was used as the reference electrode and
of TTF by H2O, photoreduction TTF+c in the presence of H2O, platinum wire as the counter electrode. All solutions were
and evidence that TTF2+ is not stable under ambient conditions degassed with the nitrogen gas saturated with the CH3CN
since it reacts with water to generate TTF+c and HTTF+.
vapour for 10 min prior to commencing voltammetric
measurements. 0.1 M Bu4NPF6 was used as the supporting
electrolyte in neat CH3CN or CH3CN–H2O mixtures.
Hydrodynamic voltammetric measurements with a rotating
disk electrode (RDE) were performed with the BAS100 electro-
chemical workstation combined with a BAS MF-2066 glassy
carbon rotating disk working electrode (A ¼ 0.0707 cm2) and the
same reference and auxiliary electrodes as used for cyclic vol-
tammetry. RDE measurements were performed in an electro-
chemical cell specially designed for small volumes.31
Bulk electrolysis experiments were used to generate the one-
and two-electron oxidized species (TTF+ and TTF2+) in a three
compartment cell, with each compartment being separated by a
ne glass frit to minimize solution mixing. A large-area GC felt
cloth (2 ꢃ 2 cm) (Tokai Carbon Japan) or a BAS GC basket net
were used as working electrodes.
2. Experimental section
Materials and chemicals
CH3CN (HPLC grade, Merck) was distilled from calcium hydride
and stored in a glove box until required. TTF (99%, Aldrich),
54% ethereal HBF4 (Aldrich), 2-propanol (BDH), chloroform
and acetone were used as supplied by the manufacturer. Tet-
rabutyl ammonium hexauorophosphate (Bu4NPF6, 98%) was
purchased from Wako Pure Chemical Industries, Japan and was
recrystallized from ethanol (95%) and vacuum dried prior to its
use. Universal indicator paper with a pH range of 1.0 to 11.0 was
purchased from Tokyo Roshi Kaisha, Japan.
Chemical synthesis
Unless otherwise stated, potentials are referenced to the
ferrocene/ferrocenium (Fc0/+) couple. Thus, the measured
potential vs. Ag/Ag+ was converted to the ferrocene scale via
measurement of the reversible formal potential of the Fc0/+
(HTTF)BF4 was synthesized as follows using a modied litera-
ture procedure.26 TTF (2.5 mmol) was dissolved in 10 mL of
anhydrous CHCl3 while under a nitrogen atmosphere. 40 mL of
a 54% ethereal HBF4 (3.0 mmol) was dissolved in 10 mL of
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redox couple vs. Ag/Ag+ under the relevant conditions. The at a scan rate of 100 mV sꢀ1 and in both solutions. This value
potential of the Ag/Ag+ reference electrode is ꢀ85 mV vs. Fc0/+
.
is higher than that theoretically predicted value of about 60 mV
Evolution of oxygen was detected using a Clark electrode expected for a reversible process which implies that a small
(Unisense, Aarhus, Denmark). This membrane-based sensor level of uncompensated ohmic drop (iRu) is present, as also
was calibrated daily using the two-point method recommended found for the well known reversible oxidation of Fc under the
by Unisense (0% and 100% air saturation). For photochemical same conditions.33 The oxidation peak potentials (Epox1, Epox2),
experiments with oxygen concentration monitoring, the solu- reduction peak potentials (Erped1, Epred2) and midpoint potentials
tion was initially purged with nitrogen for 5 minutes to remove (Em1, Em2) in the absence and presence of 10% H2O are
oxygen and then sealed to avoid the introduction of oxygen gas summarized in Table S1.† Addition of water to the CH3CN
from the atmosphere.
solution shis these potentials slightly to more negative values
In photochemical experiments, light from a Polilight PL6 (vs. Ag/Ag+), probably due to the introduction of a liquid
(l ¼ 275–750 nm) Xe source (Ron) was used. The total power junction potential. A similar negative shi is observed for
output of the lamp at the end of the liquid light guide was 7 W. oxidation of ferrocene in the presence of H2O (Fig. S1†).
Full information about the light source are available in the Interestingly, for TTF in the presence of 10% H2O, although
manual.32 In these experiments, the light source was positioned the electrochemical reversibility of the two oxidation processes
to one side of the soda glass vial at a distance of 10 cm from the is retained, an additional small irreversible oxidation reaction
wall–solution interface. Care was taken to avoid exposure of the (Eopx3 ¼ 950 mV) is evident, lying at more positive potentials
Clark electrode to the light source. The photochemical reaction than for the TTF+c/2+ process (Fig. 1). This new process is
takes place with this conguration gives rise to a heterogeneous located at the same potential as found for oxidation of HTTF+
solution with maximum generation of products being achieved (as discussed below), which implies that water acts as a proton
in the solution located near the cell wall.31
donor to convert a small amount of TTF to HTTF+ (eqn (5))
which is then oxidized as described in eqn (6) and (7).
Commonly, acid–base reactions are rapid and in equilibrium
on the voltammetric time scale. However, in the present situ-
ation, the structures of TTF and HTTF+ differ substantially (see
the structures in eqn (3)),26 so the acid–base chemistry is
expected to be kinetically slow, allowing the observation of a
separate oxidation processes when mixtures of both species are
present. Since the cyclic voltammetric data at Au, Pt and ITO
electrodes are similar to that at a GC electrode (Fig. S2†),
surface interaction is not believed to be signicant here,
although changes in peak current are observed associated with
variation of electrode area.
3. Results and discussion
Voltammetry of TTF in CH3CN–H2O mixtures
Cyclic voltammograms for the oxidation of 1 mM TTF in
CH3CN (0.1 M Bu4NPF6) in the absence and the presence of
10% H2O are shown in Fig. 1; obtained with a scan rate at 100
mV sꢀ1 at a GC electrode. In the absence of H2O, TTF exhibits
two diffusion controlled, one-electron, reversible oxidation
processes described by eqn (1) and (2), as well documented in
the literature.2,22–25 The midpoint potentials (Em), calculated as
the average of the oxidation (Eopx) and reduction (Epred1) peak
potential are ꢀ74 and 311 mV vs. ferrocene (Fc0/+) in CH3CN
and when 10% water is present shi to ꢀ94 and 285 mV,
respectively. Em is approximately equal to the reversible formal
potential E0f . The peak-to-peak separation (DEp) for oxidation
and reduction processes are about 80 mV for both oxidations
TTF + H2O # HTTF+ + OHꢀ
HTTF+ # HTTF2+ + e
HTTF2+ / Product(s)
(5)
(6)
(7)
Steady-state voltammograms obtained with a carbon ber
microelectrode (12 mm diameter) in the absence and in the
presence of 10% H2O (Fig. S3†) conrms that the two TTF
oxidation processes are electrochemically reversible, as differ-
ences in the potentials (E3/4 ꢀ E1/4) are 60 mV. Thus the half-
wave potentials (E1/2) are almost identical to the mid-
potentials (Em) obtained from cyclic voltammograms at a
3 mm diameter GC electrode. Diffusion coefficients of TTF in
CH3CN in the absence and in the presence of 10% H2O were
calculated using these steady state voltammograms and are
2.1 ꢃ 10ꢀ5 and 2.0 ꢃ 10ꢀ5 cm2 sꢀ1, respectively. These values
are also in excellent agreement those derived from the transieꢀn5t
voltammetry using the Randles–Sevcik equation33 of 2.2 ꢃ 10
(absence of water) and 2.1 ꢃ 10ꢀ5 cm2 sꢀ1 (10% water present)
Fig. 1 Cyclic voltammograms for the oxidation of 1 mM TTF in CH3CN and also with the value of DTTF of 2.2 ꢃ 10ꢀ5 cm2 sꢀ1 reported by
Norton et al. in CH3CN.34
(0.1 M Bu4NPF6) obtained with a 3 mm diameter GC electrode using a
scan rate of 100 mV sꢀ1 in the absence and presence of 10% H2O.
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Photochemical oxidation of H2O by TTF+c
In the absence of water, both TTF+c and TTF2+ can be quanti-
tatively produced by oxidative bulk electrolysis of neutral TTF
(1 mM) in CH3CN (0.1 M Bu4NPF6) using an applied potential
(Eappl) of 120 mV and 620 mV (vs. Fc0/+), respectively.35 This
outcome is conrmed by noting the change in the relative
position of the zero current in the GC rotating disk electrode
(RDE) voltammograms before and aer stepwise bulk electrol-
ysis to generate a persistent solution of the red coloured TTF+c
or the yellow coloured TTF2+ (Fig. S4†).
In the presence of 10% H2O, in the dark (Fig. 3), the time
independent zero current position and limiting current
magnitude conrmed the stability of TTF+c. However, cyclic
voltammograms for a 1 mM TTF+c solution in CH3CN contain-
ing 10% H2O (0.1 M NBu4PF6) change aer exposure to light
(Fig. 2). Irradiation with white light from a Xe source (l ¼ 275–
750 nm) for 2 min, resulted in an irreversible oxidation process
appearing at about 950 mV, which is assigned to the oxidation
of HTTF+. Furthermore, aer 10 minutes irradiation, the
apparent “pH” of the solution (determined using pH paper)
dropped to ꢄ3–4 indicating that the solution became acidic.
The voltammetric parameters for the TTF+c redox processes, in
the presence and absence of H2O and/or light are summarized
in Table S2.†
Fig. 3 Voltammograms for 1 mM TTF+c (obtained by bulk electrolysis)
in CH3CN and in (90 : 10) (v/v) CH3CN–H2O (0.1 M Bu4NPF6) before
and after irradiation with white light for designated periods of time.
These data were obtained with a 3 mm diameter GC RDE (rotation rate
1000 rpm).
unstable as shown in Fig. 2 and 3. Fig. 3 shows the steady state
voltammograms resulting from a TTF+c solution in CH3CN with
10% H2O before and aer exposure to white light. In the dark,
the relative position of the zero current remains unchanged and
similar to that obtained in neat CH3CN, conrming that TTF+c
is stable in the presence of 10% H2O. However, upon exposure
to white light, the relative position of zero current shis to a
value that reveals the solution now contains TTF as well as
TTF+c. Increasing the irradiation time, the concentration of
TTF+c present in the solution decreases to a negligible value
while TTF and HTTF+ increases, concomitantly. Thus, this
observation indicates that TTF+c is photoreduced in the pres-
ence of H2O, which also implies that H2O is oxidized in the
counter redox reaction. A reaction scheme for this photoactivity
is proposed in eqn (8)–(10). If this scenario is correct, in the
presence of light, TTF+c can oxidize H2O directly to oxygen.
The UV-visible spectrum of the red coloured, electrochemi-
cally generated TTF+c solution in CH3CN (0.1 M Bu4NPF6) is
shown in Fig. S5.† Absorption maxima are observed at wave-
lengths (lmax) of 355 nm and 273 nm for TTF2+, at 335, 430 and
579 nm for TTF+c, while TTF exhibits two absorption bands with
lmax at 302 and 317 nm. These spectra are also in good agree-
ment with those reported in the literature.6,36,37
Irradiation of the TTF+c solution in neat CH3CN using white
light for 10 min, had no effect on the UV-visible spectrum,
implying TTF+c is stable in dry CH3CN in the presence of light.
However, in the presence of water, TTF+c is photochemically
hv
ƒƒƒƒƒ!
þ
4 TTFþ
4 ðTTF Þ*
(8)
c
c
4(TTF+c)* + 2H2O / 4TTF + O2 + 4H+
4TTF + 4H+ / 4HTTF+
(9)
(10)
To conrm that oxygen is a product of irradiation of CH3CN–
TTF+c solutions that contained H2O, a Clark electrode was used.
The experiment required removal of adventitious oxygen by
rigorous degassing with nitrogen and the reaction cell was
tightly sealed to ensure that no atmospheric oxygen could be
introduced. The data obtained using the Clark electrode (Fig. 4)
shows the oxygen percentage relative to the solubility limit, as a
function of irradiation time for a 1 mM TTF+c in neat CH3CN
solution and with water present. In the absence of H2O, the
oxygen concentration remains close to zero even when the 1
mM TTF+c solution is illuminated with white light (275–750
nm). Similarly, no oxygen is detected or change in the intensity
of the red coloured TTF+c solution is observed, with or without
Fig. 2 Cyclic voltammograms obtained with a 3 mm diameter GC
electrode at a scan rate of 100 mV sꢀ1 showing the (TTF+c/2+) oxidation
and (TTF+c/0) reduction processes derived from 1 mM TTF+c solutions
in CH3CN and mixed CH3CN–H2O (90 : 10%) solvent media (0.1 M
Bu4NPF6) in the dark and after irradiation with white light (l ¼ 275–750
nm) for 2 min.
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Fig. 5 Steady state voltammograms for 1 mM TTF2+ generated by bulk
electrolysis in CH3CN (0.1 M Bu4NPF6) in the absence and in the
presence of 10% H2O obtained at a 3 mm GC electrode RDE (rotation
rate 1000 rpm).
Fig. 4 Oxygen percentage as a function of time for 1 mM TTF+c in
CH3CN (0.1 M Bu4NPF6) with or without added H2O in the dark, under
exposure to irradiation with white light (at 275–750 nm) or 425 nm
visible light.
10% (v/v) H2O to the CH3CN solution containing 1 mM TTF2+
,
10% H2O, in the dark. In contrast, with H2O present and under
irradiation with white light, an increase in oxygen concentra-
tion was clearly detected and the solution colour changed from
red to yellow. Together these outcomes are all consistent with
photo-reduction of TTF+c to the neutral TTF and the photo-
oxidation of H2O to O2. This photochemical irradiation of
TTF+c was repeated using 425 nm visible light, which lies close
to the value of lmax 430 nm for the TTF mono cation. Under
these conditions, evolution of oxygen was still detected, but the
rate of increase was less than that with white light. Additionally,
it was found that the rate of oxygen generation is enhanced in
the presence of 20% H2O relative to that found with 10% H2O.
In summary, the irradiation TTF+c leads to the evolution of
oxygen and a change in solution colour in the presence of
aqueous CH3CN and together with the voltammetric data
are consistent with the reaction scheme summarized in
eqn (8)–(10).
resulted in the light yellow solution immediately turning dark
purple-red, the colour expected for a TTF+c solution. The
apparent “pH” of this solution was checked aer 5 minutes
(using of pH paper), indicating a value of ꢄ3, hence the solution
was now acidic. A control experiment with no TTF2+ present and
containing 10% H2O (v/v) in CH3CN (0.1 M Bu4NPF6) showed the
apparent “pH” value remained in the 6–7 range. Thus, the
instability of TTF2+ in presence of H2O generates protons.
The new oxidation process found in the transient and steady
state voltammograms of TTF2+ at ꢄ950 mV (Fig. 5 and S6†) are
consistent with the reaction between TTF2+ and H2O generating
protonated HTTF+ species. The steady state voltammetric condi-
tions show the current for the TTF2+/+c and TTF+c/0 reduction
processes decrease to a very small level. Using the position of zero
current as a starting point, the residual material is TTF+c (ꢄ20%)
and nearly 80% of the TTF2+ has been converted to HTTF+
(eqn (11)).
2TTF2+ + 2H2O / 2HTTF+ + O2 + 2H+
(11)
Instability of TTF2+ in the presence of water
As described above, TTF+c reacts with water when exposed to
light to give TTF and O2. In contrast, the dication TTF2+ reacts
with H2O rapidly, even in the dark, as briey noted in a paper by
Siedle et al.38 In orderto explorethis process inmore detail, TTF2+
was quantitatively generated by exhaustive oxidation of a 1 mM
TTF in CH3CN (0.1 M Bu4NPF6) at 620 mV (vs. Fc0/+) in a carefully
sealed electrochemical cell placed in a dry box to avoid the
introduction of water. During this bulk electrolysis experiment,
the initial light-yellow TTF solution changed colour initially to
the dark red (TTF+c) and then to a light yellow (TTF2+) solution.
Steady state voltammetry of the TTF2+ solution was undertaken
10 minutes aer bulk electrolysis at a GC electrode. The position
Once again using a sealed reaction cell, a Clark electrode was
used to conrm oxygen evolution from TTF2+ solutions in which
water was present. The data in Fig. S7† show no oxygen evolu-
tion for TTF2+ in neat CH3CN compared with signicant oxygen
evolution for 1 mM TTF2+ CH3CN solution aer dilution with
10% H2O, under a nitrogen atmosphere and exposed to light.
The colour of the TTF2+ solution was also observed to change
from yellow to red in the presence of water, also consistent with
formation of HTTF+.
Electrochemical characterization of HTTF+
of zero current conrmed that complete oxidation of TTF to Previous studies by Giffard et al.26 noted that HTTF+ reverted
TTF2+ was achieved (Fig. 5). The solution was taken from the back to neutral TTF once dissolved in aqueous media, but they
anaerobic conditions and cyclic voltammetry detected an addi- did not provide any mechanistic details. We used voltametric
tional oxidation process at ꢄ950 mV (Fig. S6†) increased with studies on (HTTF)BF4 to obtain a more detailed understanding
time over a few minutes together with a change of the solution of the reactions of TTF, TTF+c and TTF2+ with H2O in the dark
colour from light yellow to orange aer ꢄ20 minutes. Addition of and on irradiation with light.
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chemistry under ambient conditions should assist the devel-
opment of practical devices derived from use of TTF based
materials.
Acknowledgements
AMB and LLM gratefully acknowledge nancial support from
the Australian Research Council. SA gratefully acknowledges
receipt a Monash Graduate Scholarship and we thank Mr
Muhammad Abdelhamid for his assistance with the gures.
References
´
1 J. L. Segura and N. Martın, Angew. Chem., Int. Ed., 2001, 40,
1372–1409.
Fig. 6 Cyclic voltammograms for 1 mM (HTTF)BF4 in CH3CN (0.1 M
Bu4NPF6) using a scan rate of 100 mV sꢀ1 at designated switching
potentials (El) obtained with a 3 mm diameter GC electrode. Cyclic
voltammograms were obtained under rigorous dry, anaerobic
conditions.
2 F. Wudl, G. M. Smith and E. J. Hufnagel, J. Chem. Soc. D,
1970, 1453–1454.
3 Y. Wang, S. Cui, B. Li, J. Zhang and Y. Zhang, Cryst. Growth
Des., 2009, 9, 3855–3858.
´
´
´
4 E. Coronado, C. Gimenez-Saiz and C. J. Gomez-Garcıa,
Cyclic voltammograms for (HTTF)BF4 in CH3CN (0.1 M
Coord. Chem. Rev., 2005, 249, 1776–1796.
Bu4NPF6) are shown in Fig. 6. In addition to both the TTF0/+
c
5 E. Coronado, J. R. Galan-Mascaros, C. Gimenez-Saiz,
C. J. Gomez-Garcia and V. N. Laukhin, Adv. Mater., 1996, 8,
801–803.
and TTF+c/2+ reversible redox processes are present, an irre-
versible oxidation process at 950 mV is well dened. There are
characteristics similar to the voltammogram in Fig. 1 following
the addition of 10% H2O to the CH3CN solution. The presence
of the reversible redox processes for TTF0/+c and TTF+c/2+
following dissolution of (HTTF)BF4 in CH3CN suggests that an
equilibrium reaction, in eqn (12), occurs.
´
6 R. Andreu, J. Garın and J. Orduna, Tetrahedron, 2001, 57,
7883–7892.
7 N. Dupont, Y.-F. Ran, S.-X. Liu, J. Grilj, E. Vauthey,
S. Decurtins and A. Hauser, Inorg. Chem., 2012, 52, 306–312.
8 C. M. Davis, J. M. Lim, K. R. Larsen, D. S. Kim, Y. M. Sung,
D. M. Lyons, V. M. Lynch, K. A. Nielsen, J. O. Jeppesen,
D. Kim, J. S. Park and J. L. Sessler, J. Am. Chem. Soc., 2014,
136, 10410–10417.
HTTF+ # TTF + H+
(12)
9 A. Amacher, C. Yi, J. Yang, M. P. Bircher, Y. Fu, M. Cascella,
M. Gratzel, S. Decurtins and S.-X. Liu, Chem. Commun., 2014,
50, 6540–6542.
Therefore the irreversible oxidation process at 950 mV is due
to the formation of HTTF2+
.
10 M. Frasconi, T. Kikuchi, D. Cao, Y. Wu, W. G. Liu, S. M. Dyar,
G. Barin, A. A. Sarjeant, C. L. Stern, R. Carmieli, C. Wang,
M. R. Wasielewski, W. A. Goddard and J. F. Stoddart, J. Am.
Chem. Soc., 2014, 136(31), 11011–11026.
4. Conclusions
We have systematically examined the voltammetric behaviour
of TTF, TTF+c, TTF2+ and HTTF+ in CH3CN (0.1 M Bu4NPF6) in 11 M. R. Bryce, J. Mater. Chem., 2000, 10, 589–598.
the presence of water and light in order to elucidate the rela- 12 J. Bigot, B. Charleux, G. Cooke, F. o. Delattre, D. Fournier,
tionship between their chemistries and photochemistry under
ambient conditions. This has resulted in identication of
J. l. Lyskawa, L. n. Sambe, F. o. Stoffelbach and P. Woisel,
J. Am. Chem. Soc., 2010, 132, 10796–10801.
several different pathways for the reaction of TTF and its cations 13 W. Sun, C.-H. Xu, Z. Zhu, C.-J. Fang and C.-H. Yan, J. Phys.
in water with or without light. TTF reacts with H2O to form
Chem., 2008, 112, 16973–16983.
HTTF+. TTF+c reacts with water to give TTF, HTTF+ and O2, 14 M. D. Halling, J. D. Bell, R. J. Pugmire, D. M. Grant and
under irradiation with light. Mechanistically, TTF+c is photo-
J. S. Miller, J. Phys. Chem. A, 2010, 114, 6622–6629.
reduced to TTF and H2O is oxidized to oxygen, releasing 15 D. M. Guldi, L. Sanchez and N. Martın, J. Phys. Chem. B, 2001,
protons in these photochemical reactions. Monitoring the 105, 7139–7144.
irradiated TTF+c solution with a Clark electrode conrms the 16 T. Akutagawa, K. Kakiuchi, T. Hasegawa, S.-i. Noro,
´
´
evolution of oxygen in CH3CN solution containing 10% H2O.
T. Nakamura, H. Hasegawa, S. Mashiko and J. Becher,
Angew. Chem., Int. Ed., 2005, 44, 7283–7287.
Conrmation of the instability of TTF2+ under ambient condi-
tions revealed that the TTF2+ dication reacts with H2O to 17 J. M. Williams, A. J. Schultz, U. R. S. Geiser, K. D. Carlson,
generate TTF+c and HTTF+. Together, the photochemistry and
A. M. Kini, H. H. Wang, W.-K. Kwok, M.-H. Whangbo and
the voltammetric measurements reveal important reaction
J. E. Schirber, Science, 1991, 252, 1501–1508.
pathway differences for TTF, TTF+c, TTF2+ and HTTF+ redox 18 Z. Yu, H. Tian, E. Gabrielsson, G. Boschloo, M. Gorlov, L. Sun
chemistries in the presence of H2O and light. Knowledge of the
and L. Kloo, RSC Adv., 2012, 2, 1083–1087.
49794 | RSC Adv., 2014, 4, 49789–49795
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Paper
RSC Advances
19 J. Ferraris, D. O. Cowan, V. Walatka and J. H. Perlstein, J. Am. 28 M. Giffard, M. Sigalov, V. Khodorkovsky, A. Gorgues and
Chem. Soc., 1973, 95, 948–949. G. Mabon, Synth. Met., 1999, 102, 1713.
20 A. J. Olaya, P. Ge, J. r. m. F. Gonthier, P. Pechy, 29 M. Giffard, A. Gorgues, A. Riou, J. Roncali, T. P. Nguyen,
´
C. m. Corminboeuf and H. H. Girault, J. Am. Chem. Soc.,
2011, 133, 12115–12123.
J. Garın, S. Uriel and P. Alonso, Synth. Met., 1995, 70,
1133–1134.
21 J. C. Ndamanisha, X. Bo and L. Guo, Analyst, 2010, 135, 621– 30 M. Giffard, P. Alonso, J. Garin, A. Gorgues, T. P. Nguyen,
629.
P. Richomme, A. Robert, J. Roncali and S. Uriel, Adv.
Mater., 1994, 6, 298–300.
22 J. M. Spruell, A. Coskun, D. C. Friedman, R. S. Forgan,
A. A. Sarjeant, A. Trabolsi, A. C. Fahrenbach, G. Barin, 31 G. Bernardini, C. Zhao, A. G. Wedd and A. M. Bond, Inorg.
´
W. F. Paxton, S. K. Dey, M. A. Olson, D. Benıtez,
Chem., 2011, 50, 5899–5909.
E. Tkatchouk, M. T. Colvin, R. Carmielli, S. T. Caldwell, 32 Ron Polilight PL6 manual, http://www.ron.com.au.
G. M. Rosair, S. G. Hewage, F. Duclairoir, J. L. Seymour, 33 A. M. Bond, Broadening Electrochemical Horizons: Principles
A. M. Z. Slawin, W. A. Goddard, M. R. Wasielewski,
and Illustration of Voltammetric and Related Techniques,
G. Cooke and J. F. Stoddart, Nat. Chem., 2010, 2, 870–879.
Oxford University Press, 2002.
23 A. Y. Ziganshina, Y. H. Ko, W. S. Jeon and K. Kim, Chem. 34 J. D. Norton, W. E. Benson, H. S. White, B. D. Pendley and
Commun., 2004, 806–807. H. D. Abruna, Anal. Chem., 1991, 63, 1909–1914.
24 Q. Li, C. Zhao, A. M. Bond, J. F. Boas, A. G. Wedd, 35 A. M. Bond, K. Bano, S. Adeel, L. L. Martin and J. Zhang,
B. Moubaraki and K. S. Murray, J. Mater. Chem., 2011, 21,
5398–5407.
ChemElectroChem, 2013, 1(1), 99–107.
36 L. Huchet, S. Akoudad, E. Levillain, J. Roncali, A. Emge and
¨
25 S. J. Shaw, F. Marken and A. M. Bond, Electroanalysis, 1996,
8, 732–741.
26 M. Giffard, P. Frere, A. Gorgues, A. Riou, J. Roncali and
L. Toupet, J. Chem. Soc., Chem. Commun., 1993, 944–945.
27 Y. Kobayashi, M. Yoshioka, K. Saigo, D. Hashizume and
T. Ogura, J. Am. Chem. Soc., 2009, 131, 9995–10002.
P. Bauerle, J. Phys. Chem. B, 1998, 102, 7776–7781.
37 J.-F. Bergamini, P. Hapiot and D. Lorcy, J. Electroanal. Chem.,
2006, 593, 87–98.
38 A. R. Siedle, G. A. Candela, T. F. Finnegan, R. P. Van Duyne,
T. Cape, G. F. Kokoszka, P. M. Woyciejes and J. A. Hashmall,
Inorg. Chem., 1981, 20, 2635–2640.
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